1. Field of the Invention
This invention relates to an anti-rolling apparatus for reducing a rolling of an object whose rolling is to be reduced, and more particularly to dynamic vibration reducer type anti-rolling apparatus constructed so as to reduce a rolling of the object by a movable weight which reciprocates along a rail. The objects whose rolling is to be reduced include ships in stoppage condition, marine structures floating on the sea or water such as barge, and structure hoisted in the air such as lift, gondola and the like.
2. Description of the Related Art
Since before as the anti-rolling apparatus for reducing a rolling of the object, an active type apparatus using an actuator and a passive type apparatus using a dynamic vibration reducer principle have been known. The active type apparatus detects a rolling of the object by means of a sensor and vibrates a movable weight by means of an actuator. The vibration of the movable weight is controlled in terms of phase so as to reduce the rolling of the object. Further, some type apparatus produces an anti-rolling effect by using a torque depending on gyro effect.
On the other hand, the passive type apparatus using the dynamic vibration reducer principle is simple in structure because it does not utilize an actuator for driving the movable weight, and is widely applicable because it does not consume much electricity.
Referring to FIG. 1, an example of a conventional anti-rolling apparatus using the dynamic vibration reducer principle will be described. This example was disclosed in Japanese Patent Application No. H8-15428 filed in Jan. 31, 1996 by the same applicant as this invention.
This anti-rolling apparatus comprises a rail member 511 curved in a circular shape, a movable weight 512 capable of moving freely along the rail member 511 and supporting members 513A, 513B located on both sides. Horizontal shafts 511A, 511B are mounted on both ends of the rail member 511 and the horizontal shafts 511A, 511B are rotatably supported by bearings (not shown) in the supporting members 511A, 511B.
The supporting members 513A, 513B are mounted vertically on a predetermined base 514 of a marine structure. Thus the horizontal shafts 511A, 511B are parallel to the base 514. As shown in the Figure, x-axis is set along the horizontal shafts 511A, 511B on a plane parallel to the base 514, y-axis is set perpendicular to the x-axis and then z-axis is set perpendicular to the base 514.
This anti-rolling apparatus is so constructed as to reduce a rolling around a rotary axis parallel to the y-axis of the marine structure. When the marine structure rolls around the rotary axis parallel to the y-axis, the movable weight 512 reciprocates along the rail member 511. The movable weight 512 reciprocates on the circular path along the rail member 511. A component of force of the gravity becomes a restoring force for the reciprocating motion. A center of the vibration of the movable weight 512 is a center of the circular path, which is located at the lowest point.
The vibration of the marine structure is reduced by the reciprocating motion of the movable weight 512. For the anti-rolling apparatus to function effectively, the reciprocating motion of the movable weight 512 needs to have the same oscillation cycle as the oscillation cycle of a marine structure and further a phase deviated by only a predetermined angle or displacement with respect to the phase of the marine structure.
Generally, the oscillation cycle of the marine structure is governed by the natural oscillation cycle of the marine structure. The natural oscillation cycle of the marine structure is determined depending on a structure, mass, gravity center, and the like of the marine structure, and differs depending on the marine structure. If freight or the like is changed, the mass, gravity center and the like are changed, so that the natural oscillation cycle is also changed.
On the other hand, the oscillation cycle of the movable weight is governed by the natural oscillation cycle of the movable weight 512. The natural oscillation cycle of the movable weight 512 is determined depending on the mass, motional path and the like of the movable weight 512. To obtain a desired anti-rolling effect, it is necessary to make the natural oscillation cycle of the movable weight 512 substantially match with the natural oscillation cycle of the marine structure.
The anti-rolling apparatus shown in FIG. 1 is so constructed that the natural oscillation cycle of the movable weight 512 in the anti-rolling apparatus can be adjusted. Even if the freight or the like on the marine structure is changed, so that the natural oscillation cycle is changed, the desired anti-rolling effect can be obtained by adjusting the natural oscillation cycle of the movable weight 512 in the anti-rolling apparatus.
In this example, the rail member 511 can be rotated around the horizontal shafts 511A, 511B. Consequently, the movable weight 512 moves along the rail member 511 on a plane inclined relative to the x-z plane.
An external force originating from a vibration of the marine structure and gravity act upon the movable weight 512. A force which contributes for the motion of the movable weight 512 is a component in the direction of motion of the movable weight 512 or a component in the direction of tangent line on a central axis of the rail member 511.
The restoring or stability force of the reciprocating motion of the movable weight 512 is based on the gravity. For example, assuming that an angle formed by a tangent line on the central axis of the rail member 511 relative to vertical line is .alpha., the restoring force is mgcos .alpha..
When the rail member 511 rotates around the horizontal shafts 511A, 511B, cos .alpha. is reduced thereby reducing the restoring force. As a result, the natural oscillation cycle of the movable weight 512 increases.
Therefore, when the natural oscillation cycle of the marine structure is increased due to a change of freight or the like, the rail member 511 is rotated around the horizontal shafts 511A, 511B so as to increase the natural oscillation cycle of the movable weight 512, thereby achieving a desired anti-rolling effect.
The conventional anti-rolling apparatus shown in FIG. 1 utilizes the rail member 511 which is curved in a circular shape. Production of the curved rail member 511 at a high precision is very difficult, therefore mass production thereof could not be carried out. To process the rail member 511 in accurate circular shape, the production cost increases.
In the conventional anti-rolling apparatus, because the rail member 511 formed in a circular shape is used, a volume occupied by the anti-rolling apparatus, particularly a portion for incorporating the rail member 511 and the movable weight 512 are enlarged. Particularly when this apparatus is loaded on a small size ship or the like, sometimes it could not be loaded thereon.
In the conventional anti-rolling apparatus, the natural oscillation cycle of the movable weight 512 could not be reduced although it could be increased.
A conventional anti-rolling apparatus using the dynamic vibration reducer principle will be described with reference to FIG. 2. This anti-rolling apparatus comprises a rail member 520 having a rail face 521 curved in a circular shape, a movable weight 522 capable of moving freely on the rail face 521 and an electric conductor member 530 curved in a circular shape parallel to the rail face 521.
The movable weight 522 has a pair of wheels 523 each on the front and rear portions. On both ends of the rail face 521 are provided stoppers 521A, 521B for specifying a stroke of the movable weight 522.
A construction and operation of a magnetic damper provided on the anti-rolling apparatus shown in FIG. 2 will be described with reference to FIG. 3. The movable weight 512 has a concave portion 522A, so that it has U-shaped cross section. On an internal face of this concave portion 522A are mounted a pair of permanent magnets 532, 532. As shown in this Figure, the permanent magnets 532, 532 are disposed on both sides of a sheet-like electric conductor member 530 with a slight gap.
The electric conductor member 530 and the permanent magnets 532, 532 form the magnetic damper. The electric conductor member 530 is made of electric conductive material such as copper and the movable weight 522 is made of metal having a small magnetic resistance such as iron. As shown by an arrow M, a magnetic path passing the movable weight 522, the permanent magnets 532, 532 and the electric conductor member 530 on the U-shaped cross section, is formed.
Magnetic flux generated by the permanent magnets 532, 532 passes through the electric conductor member 530. When the movable weight 522 moves along a rail surface 521, magnetic flux passing the electric conductor member 530 is moved, so that eddy current is generated in the electric conductor member 530 sandwiched by the permanent magnets 532, 532 because of Fleming's rule. Because of this eddy current, a braking force is applied to the movable weight 522 supporting the permanent magnets 532, 532. This braking force acts as a damping force for the reciprocating movable weight 522.
The magnetic damper has the following characteristics.
(1) The damping force is accurately proportional to a speed of motion of the movable weight 522. (2) There is no mechanical contacting component thereby causing no friction, and therefore an excellent durability is ensured. (3) The damping force less depends on temperature.
A restoring force applied to the movable weight 522 will be described with reference to FIG. 4. Assume that if the movable weight 522 reciprocates along the rail face 521, the gravity center G of the movable weight 522 reciprocates on a circle in which a center thereof is O' and a radius is R. As shown in the Figure, the lowest point of a tracing of the gravity center G is designated as an origin O and x-axis is set horizontally and y-axis is set vertically. Further, z-axis is set perpendicular to the x-axis and y-axis (perpendicular to the paper sheet). This anti-rolling apparatus is so constructed as to reduce a rolling around a rotary axis parallel to the z-axis of the object.
When the movable weight 522 reciprocates along the rail face 521, a component in the direction of tangent line applied to the movable weight 522 acts as a restoring force. For example, assuming that a displacement of the gravity center G of the movable weight 522 is x and an angle formed by a radius O'G of the circle and a vertical line (Y-axis) is .alpha., a component of the gravity in the direction of tangent line is mgsin .alpha.=(mg/R)x and proportional to the displacement x of the movable weight 522.
A restoring force (mg/R)x caused by the gravity and a damping force by the magnetic damper act on the movable weight 522. Therefore, an equation of motion of the movable weight 522 can be expressed as follows. EQU m(d.sup.2 x/dt.sup.2)+C(dx/dt)+kx=P (1)
where k=mg/R, C is a damping coefficient by the magnetic damper, and P is an external force caused by oscillation of the object.
Generally, for the anti-rolling apparatus to generate an optimum anti-rolling operation for an object whose oscillation is to be reduced, the vibration cycle of the movable weight 522 needs to coincide with that of the object and at the same time, the oscillations of both need to be displaced with respect to each other by a predetermined difference of phase. For example, if an oscillation angle of the object is increased so that the movable weight 522 strikes stoppers 521A, 521B on both ends, a relation in phase between both is deteriorated so that a desired anti-rolling effect cannot be obtained. Thus, it is necessary to limit a stroke of reciprocation of the movable weight 522 for the movable weight 522 not to strike the stoppers 521A, 521B.
For the movable weight 522 not to strike the stoppers 521A, 521B, the damping force by the magnetic damper is increased sufficiently so as to limit the stroke. However, if the damping force of the movable weight 522 is increased, when a rolling angle of the object is small, a desired anti-rolling effect cannot be obtained.
Thus, it is desirable that when the rolling angle of the object is large, the damping force is sufficiently large for the movable weight 522 not to strike the stoppers on both ends and when the rolling angle of the object is small, the damping force is sufficiently small so as to obtain a sufficient anti-rolling effect for the object.
Generally, the damping force by the magnetic damper is proportional to a speed of the movable weight 522. Thus, the speed of the movable weight 522 is decreased on both ends of the oscillation thereof, so that the damping force is small. Because the speed of the movable weight 522 in the vicinity of the origin O is increased, the damping force is large.
However, the magnetic damper used in the conventional anti-rolling apparatus shown in FIGS. 2, 3 could not adjust the damping force depending on the rolling angle of the object. For example, the damping coefficient C in the second term of the left part of an expression shown in Expression 1 is constant.
In an anti-rolling apparatus of passive dynamic vibration reduction type, an inertia force applied to the movable weight 522 varies depending on an installation height, so that the anti-rolling effect is changed. For example, as the installation height for the anti-rolling apparatus increases with respect to the rolling center of the object, the inertia force acting on the movable weight 522 also increases. As the installation height for the anti-rolling apparatus decreases, the inertia force acting on the movable weight 522 also decreases.
Therefore, in order to prevent the movable weight 522 from striking the stoppers 521A, 521B or limit the stroke thereof, it is necessary to increase the damping force as the installation height for the anti-rolling apparatus is increased with respect to the rolling center of the object and decrease the damping force as the installation height of the anti-rolling apparatus is decreased.
However, in the magnetic damper used in the conventional anti-rolling apparatus shown in FIGS. 2, 3, even if the installation height for the anti-rolling apparatus differs, the damping force by the magnetic damper cannot be adjusted and therefore is constant.